Self-selected parent members of the National Sudden Infant Death Foundation and the Guild for Infant Survival, who had experienced one or more sudden infant death (SID) episodes, provide the data used in this investigation. The data are acquired by means of a mailed questionnaire designed especially for eliciting associations pertinent to assessing the role of heredity in the epidemiogenesis of SID. Only infants of Caucasian parents less than one year of age who died elsewhere than in hospital and were diagnosed at autopsy as SID have been used as probands in analyses to date. Incidence rates among first cousins ranged between 2-3 per 1,000 live births--an unremarkable risk estimate. Concordance was similar for identical and fraternal twin pairs. The birth order distribution of propositi differs markedly from that postulated under a genetic hypothesis. These results are highly preliminary and tentative and will be confirmed or refuted as additional subject families are recruited during the third year of the grant period.